Think of a cheetah zooming over some African veldt in pursuit of an antelope. At 60 mph, the elegant cat takes surface undulations of all kinds smoothly in stride -- dry stream beds, berms and mounds, the wallowing holes of big bovines -- and keeps its eyes leveled at its target with single-minded efficiency. The unfortunate prey swerves desperately, but the sleek feline extends the two legs on the outside of the turn, contracts those on the inside, and makes this sudden directional correction perfectly when you would expect it to tip over and roll from the centrifugal force of such fantastic velocity. If you were riding on its back during the chase (not a widely recommended pastime), you'd experience G forces during turns, but up-and-down movement would be gentle, almost the stately lift and fall of a ship over wide swells.
Now if you could only get a car to do that, actively compensating for bumps and curves rather than just dumbly reacting, you ought to really have something. And that's the active suspension concept, currently a hot topic in automotive circles. The basic idea is to use a computer, sensors, a pump, and hydraulic cylinders at each corner to get the car to do things never before possible, such as leaning into a curve like a motorcycle, compensating for the dive of braking and the squat of acceleration, and raising and lowering its wheels individually to make pavement imperfections seem to disappear. It's even been suggested that a single tire could be lifted off the road for changing.
Westinghouse did some early research on this back in the sixties, and Lotus, GM, Volvo and others have built prototypes, but Nissan was the first to actually put this profoundly different way of getting a car to interact with the road into production. A $4,000 option on the Infiniti Q45, it's been available in Japan for over a year now (Toyota soon followed with 300 Celicas that use a roughly similar system), and 90% of Q45 home-market buyers are ordering it. In the U.S., demand averaged 15% in '91. While that doesn't make it exactly the most numerous thing out there, it indicates that a new techtrend is under way that you should know about.
I'd better mention here that although the company calls the set-up "Full-Active Suspension," it's really a half-way approach because its actuators can only push. There's no power in the jounce direction.
Nissan states that driving fun and comfort are two of its main engineering goals. Unfortunately, where suspension is concerned they are opposing qualities -- what enhances one (firm, responsive handling) detracts from the other (soft, smooth ride). Suspension has always been a compromise because springs, stabilizer bars, and shocks are, well, dumb. But add computers, a 15 liter-per-minute oil pump, pressure control valves, accumulators, an actuator for each wheel, sensors for speed, G's, and height and you get an integrated system that responds somewhat like a living organism.
Animal anatomy analogy
Which brings me back to the cheetah illustration. The system's designers actually studied how wildlife moves, thus enlisting evolution into vehicular research. They make a very nice anatomical comparison between that big cat and their active suspension. The function of the animal's heart is duplicated by the oil pump, the leg muscles are imitated by the hydraulic actuators, the semicircular canals of the middle ear have an equivalent in the G sensors, and the brain's job is handled by the electronic controller. That's all very clever, but nobody's ever going to get a mechanical/electronic device to work as well as even the most rudimentary creature, much less the graceful cheetah. The best that can be said of this or any experimental active suspension system is that it aims in that general direction and crosses the line from dumb to smart.
The force and the fluid
Mounted in tandem with the power steering pump is the prime mover of the operation, a cam-driven plunger pump with six cylinders arranged radially. The force required to drive the unit is about what you'd expect -- three horsepower at 30 mph. That's not enough to slow down the 278 hp, 274 c.i. DOHC aluminum V8 noticeably, but along with the 132 lb. weight penalty, it costs about one mpg (not so great considering that the car already carries the guzzler tax). The pressure produced, all 2,500 psi of it, exits through one-way check valves, and pulsations are damped by two gas-charged accumulators. Four gallons of synthetic oil is the medium of power transmission.
Next comes the multivalve unit, which contains a filter and main check, main relief, flow control, pilot-controlled check, and fail-safe valves. The assembly's function is basic system-wide pressure management.
A free-piston gas-charged main accumulator is mounted at each end of the vehicle. It serves as a reservoir of pressurized oil, which provides extra flow when needed, and also maintains ride height when the engine's shut down.
One front and one rear pressure control unit meters fluid force to each individual wheel actuator. Responsive pilot-type proportional electromagnetic fluid valves containing solenoids, spools, and pintle valves handle the switching.
The muscles of the system are the hydraulic power cylinder actuators, one at each of the four corners. Sub-accumulators and damping valves included in the actuator assemblies absorb the high frequency vibrations of the springs.
Springs? Well, Nissan's engineers have faith in the system, but they're not foolhardy. So, coil springs haven't been abandoned altogether. Each wheel gets one, albeit only half as strong as those you'd find in a conventional suspension. They lessen the load on the hydraulics and keep the car suspended above terra firma should the system experience a catastrophic blowout.